The present invention is related to fiber optic technology and, more particularly, to fiber optic couplers for single-mode optic fibers.
Most fiber optic networks today use single-mode optical fibers over which an optical, i.e., light, signal is transmitted over a particular wavelength. Such networks typically have numerous couplers by which a signal on a single fiber is distributed to many fibers. In a typical coupler a single input fiber joins two output fibers to form a 1.times.2 coupler, or two input fibers join two output fibers to form a 2.times.2 coupler. Other combinations are possible.
It is highly desirable for all couplers that the distribution of the signal from an input fiber to the output fibers be substantially independent of the signal's wavelength. Semiconductor lasers which are used to generate the signals on the network are fairly noisy. Light signals which are no more precise than .+-.30 nm of a defined wavelength are typical. Thus the strength of a signal on an arbitrary branch of a fiber optic network may become unpredictable in a fiber optic network with couplers which are relatively wavelength dependent.
Unfortunately, present fiber optic couplers are still relatively wavelength dependent. For example, the coupled power ratio, i.e., the ratio of the power from an input fiber to an output fiber, in present fiber optic couplers might be 0.5 for optical signals at 1.3 .mu.m and 1.5 .mu.m, typical wavelengths for optical fiber signals. However, for signals above and below these two values, the coupling ratio of such couplers might vary widely.
One technique for improving the wavelength independence for a coupler is explained in some detail in an article entitled, "Wavelength-Flattened Fused Coupler," D. B. Mortimer, Electronic Letters, Vol. 21, pp. 742-743, 1985. In this technique two single-mode optical fibers, one of which is pretapered, are twisted together and fused for a 2.times.2 coupler. Each fiber, both untapered and pretapered, forms an input and output fiber for the coupler. This technique has been found to flatten the power coupling curve of a light signal from an input fiber to an output fiber over differing wavelengths.
Another technique for improving the wavelength independence of a coupler is to fuse untwisted sections, one untapered and one pretapered, of optical fibers together. The fused sections are held in parallel juxtaposition to each other. Such a technique is described in U.S. Pat. No. 4,798,438, which issued on Jan. 17, 1989 to D. Moore et al.
Nonetheless, it is still desirable that the wavelength response of a fiber optic coupler be improved for a high-performance fiber optic network. Furthermore, the described techniques have proved difficult to implement in manufacturing operations for fiber optic couplers which have predictable and reliably reproducible coupled power ratios. Without predictability and reproducibility in manufacturing, the costs in manufacturing couplers are increased and inhibit the implementation of fiber optic networks.
The present invention solves or substantially mitigates these problems with a fiber optic coupler which has a greatly improved wavelength independence. The coupler can be manufactured with predictable and reproducible power coupling ratios in comparison with previous techniques.